1. Field of the Invention
The present invention relates to MRI (magnetic resonance imaging) which magnetically excites nuclear spin of an object with an RF (radio frequency) signal having the Larmor frequency and reconstructs an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation.
More particularly, the present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which can perform MRA (Magnetic Resonance Angiography) for acquiring a blood flow image without using a contrast medium.
2. Description of the Related Art
Magnetic Resonance Imaging is an imaging method which magnetically excites nuclear spin of an object set in a static magnetic field with an RF signal having the Larmor frequency and reconstructs an image based on NMR signals generated due to the excitation.
In the field of the magnetic resonance imaging, MRA is known as a method of obtaining an image of a blood flow. An MRA without administration of contrast materials is referred to as a non-contrast MRA (for example, refer to Japanese Publication of Patent Application No. 2001-252263). As the non-contrast MRA, an FBI (fresh blood imaging) method has been devised. In the FBI method, an ECG (electrocardiogram) synchronization to capture a fast blood flow pumped by a heart is performed, and thereby a blood vessel is satisfactorily depicted.
Meanwhile, MR (Magnetic Resonance) perfusion and delayed enhancement are conventionally used for examining an ischemic part and an infarcted part in the heart. In the conventional cardiac study, cardiac perfusion and delayed enhanced MR imaging are performed under a contrast-enhanced (CE) MRA method in which a patient is given a contrast medium after undergoing medicational stress or exercise stress for the perfusion study.
FIG. 1 shows a cross-section of myocardium for explaining the conventional cardiac study method that uses CE MRA. When dynamic imaging is performed by administering a gadolinium-based contrast medium to a patient, the level of the signal from “tissue A” becomes high. This is because blood flows into “tissue A” which is supplied with blood by normal blood vessels inside the myocardium as shown in FIG. 1. However, in a state in which blood vessels are dilated under medicational stress or exercise stress, a region of low signal level appears as “ischemic part B”, because blood flow volume decreases relatively due to stenosed blood vessels. Therefore, the region of the low signal level can be diagnosed as “ischemia as indicated as B”. In this way, ischemia test is also called stress perfusion and it can detect an ischemic part as a defect of vascular circulation by administering a contrast medium to a patient under medicational stress or exercise stress.
Additionally, a late delayed enhanced (LDE) technique is known as a method for the diagnosis of infarction. The LDE is a diagnosis method which allows a contrast medium to flow into the myocardium tissue, and thereby diagnoses the part without a function to wash out the contrast medium as the infarction. For example, in “infarction part C” where blood vessels are occluded as shown in FIG. 1, the contrast medium remains within the part C, because the tissue doesn't have the function to wash out the contrast medium. Thus, LDE occurs due to the residual contrast medium, and it enables the detection of “infarction part C” as a region of high signal level as compared to the normal “tissue A” where the contrast medium is washed out.
A cardiac examination is also performed in other diagnostic imaging units and its results are displayed in various display methods. For example, technology to display a myocardium layer of different cross-sections by using cardiac CT image data acquired with X-ray CT (computed tomography) apparatus is known. Also, technology to display a cross-sectional image of myocardium in a bulls-eye method by using cardiac 3-dimensional image data acquired using ultrasonograph (US) is known (for example, refer to the Japanese Publication of Patent Application No. 2006-198411 and No. 2005-531352).
However, in the conventional cardiac examination using an MRI apparatus, ischemic and infarct parts are diagnosed by performing dynamic imaging after injection of gadolinium-based contrast materials under influence of medicational stress or exercise. Therefore, imaging timing is restricted to the period where contrast medium is washed-out in the normal tissue and still in the infarction area after administering a contrast medium, otherwise sufficient contrast can not be obtained. Thus, it has a limit in terms of temporal resolution as a problem. Additionally, spatial resolution also degrades due to the restriction of time resolution. Under the aforementioned technical background, there is a problem that image quality varies and diagnosis varies among readers.
Moreover, the relationship between the gadolinium-based contrast medium and Nephrogenic Systemic Fibrosis (NSF) is concerned due to a black-box warning from FDA (Food and Drug Administration). Furthermore, in stress perfusion test, risk of medication such as adenosine and dipyridamole is also a huge concern. The aforementioned problems apply to a case in which CE MRA imaging is acquired for various imaging regions.